Well... I think you stick them down the bore and measure the gap but I can't find the details/spec anywhere in the WSM.

Can somebody help?

Or should I just replace them all regardless?

It's a 1986 16V with 100k on the clock

 

H,

i would think a ring set should be capable of covering 200k miles before considering change out but given they are now 35 years old and and your motor is apart, assuming you have 97mm bore and Roger does a complete set of Goetze [?] make for circa US$240 one has to ask what do the current set really owe you?

i took a look in my WSM for you but interestingly could not find anything about ring gap. There is a rule of thumb for such based on diameter but I cannot remember such!

 

That was my thinking. Just change them. I was thinking about the rod bearings but they look perfect and I've read some terrifying stuff from Greg so I might just put them back.

 

I consider rings to be a standard replacement part when an engine is apart.

Beyond that, we always remove the rings to clean the carbon from the pistons (ring lands get carbon and gummy oil in them (possibly leading to a stuck ring). At $27.50 per piston (our price) it's not worth the effort to keep them separate and install them back on the same piston.

And for a final reason, a new set of Goetze rings contains a more modern, better functiom, oil contol ring.

 

Dicking around with the big end shells is no joke getting the correct clearances. You should use new nuts on the conrods that you pulled apart for inspection.

Changing rings out you have the opportunity to ensure there is no carbon build up in the ring lands at the same time

 

All sounds very reasonable.

Why new nuts? Don't remember that in the WSM?

 

If your nuts had been going up and down at 6000 rpm for 35 years I reckon they would need replacing!

It is a "best practice" I have had ingrained into me since my motorcycling days and I have read of the same for car engines. No idea what the official Porsche recommendation might be but expect it would be something along those lnes - doubtless "change the rods and bolts" at a zillion dollars a pop! Chances are you will not have an issue if you use them but then how lucky do you feel?

 

Your original nuts are probably non-serrated (not exactly sure when this occurred) and were superceeded to a better nut (with different torque.)
If you have the serrated nuts, they are a one use only part, according to Porsche. This is very important, as the serrated portion of the nut is distorted when torqued.
Using them a second time will result in a different torque.
When we have a set of rods "reconditioned". I send a new set of serrated rod nuts for the machine shop.
When they come back, I throw these nuts away and replace them with new, when I assemble the engine.
Brand new updated GTS connecting rods come torqued (not taken apart when they are sized.)
These nuts also need to be discarded.

The rod bearing are also a "standard" replacement part....there will be differential wear in different places of the bearings. They are very economical.
Current Glyco rod bearings are the correct size for most 928 crankshafts....they are essentially the "red" sized rod bearings, with the most clearance.

You need to clean up your rods, torque them, and measure both bores, for size.
I've got a post about some of the work it takes to rebuild an '84 engine going:
https://rennlist.com/forums/928-foru...ata-point.html

Scroll through it and there are pictures and some text about the connecting rods out of that engine.

Try to realize that the piston pin to bushing clearance should be .0008"-.001" (depending on your machinist's idea of what is correct.)
Given that, a pin bushing that is .0005" out of round is a huge deal.

And for the "big end". you are probably going to "shoot" for somewhere near .002" total bearing clearance.
A big end that is .001" out of round (like the one in my picture) is a recipe for almost certain disaster!!!!


Try to remember: The most important part of a "rebuild" is in the tiny details.
Measuring (which I do before things go to a machine shop and when they come back) and getting a machine shop to do things correctly is the most important part of the job.
Next comes the decisions on what parts to replace and what parts can be re-used.

The disassembly, the cleaning, and the reassembly is the super easy part!

 

Interesting. Although, I'm not any less frightened.

So... I had no issues with oil pressure or any reason to suspect bottom end issues. I can work carefully but I'm not a professional engine rebuilder. Do I just replace the rings (given I had compression issues) and retain the existing rod bearings or something else. Retaining them seems (to me) to be the lower risk approach.

 

I'm certainly not the guy that can answer this question:
In my almost 60 years of building engines, I've never re-used a rod bearing....I've always been able to find or measure flaws.

Heck, when I build an engine, it is rare for me to use a complete single set of new rod bearings, without rejecting one or two, for some reason.

 

Some unsolicited advice from an old man....

If you have an issue with an engine that doesn't involve removing the engine and taking it completely apart,
that is called a "repair". A valve job is a "repair". A timing belt is a "repair".

Once the engine is removed and completely disassembled, the job is no longer a "repair", but a "rebuild".
And the effort/parts required to do that "rebuild" correctly is completely different than any "repair" you will ever do.

.....The percentage of 928 "repairs", correctly done is pathetically low....but a pretty good percentage of those engines will run again. (They usually still have issues that need to be redone.)
.....The percentage of 928 "rebuilds" that ever run again is pathetically low. Separate out the ones that run for a few hundred miles and explode, and the percentage goes from pathetically low to incredibly low.

"Do it right. Do it once. Be done with it."
Just keep saying and thinking that, over and over again!

 

Ok - let me ask the stupid question.

Is it the case that I cannot simply buy a set of Glyco bearings from my favourite supplier, chuck them in, add new nuts, torque it up and expect a good result? This is not the impression I get from your many postings and I'm sorry for digging over old ground.

Right now, I'm not confident that I have the tools or skills to measure this stuff properly. On the other hand, I have some old-school engineer friends and I doubt this is insurmountable.

 

You can measure ring gaps with a set of metric feeler gauges inside of the bore. Most of the time they will be within spec but there is still a chance they wont.

It's just good practice to measure them in case they are too tight (causes ringland failure) or too loose (excessive blowy). You can file the ones that are too tight into spec fairly easily with a ring file.

The same goes for the rod bearings. Most of the time they will be within spec, but sometimes they wont for one reason or another and that will cause issues from rod knock to a spun bearing.

You can use plastigauge to measure, it wont be as exact as using micrometers, but it will be close enough and it is fairly economical.

But really, it's worth it to spend the extra $30 in 1 time use hardware so you don't have to tear apart the engine for a much more serious repair. I'm sure you're familiar with the saying "penny wise, pound foolish"

 

Cool - but what's the measurement for the ring gap? I can't find any mention of it.

I'm more than happy to replace what needs replaced. I just want to minimise my chances of screwing it up. This does all beg the question that if I buy a set of shells and cannot get them within spec, what then? Buy another set and hope for the best I assume?

I wasn't objecting to replacing the rod nuts. I just hadn't heard about it. If that's the plan then new nuts it is.

 

^^ It's specified on the package the rings come in, otherwise it's in section 10-02 of the WSM.

Rings 1 and 2 are 0.2-0.4mm

Ring 3 (oil ring) is 0.4-1.4mm, but that is mostly made irrelevant with the new 1 piece oil control ring design.